This invention relates generally to the field of electrical measurement apparatus and, more particularly to a probe assembly for such apparatus.
In measuring changing voltage levels within circuits, it is important that the measuring instrument cause as small of effect as possible on the circuit under test. The method of connecting the instrument to the circuit under test should happen in such a way that the performance of the instrument is maintained while having either a very small, or a known effect on the circuit. If measuring changes the voltage in a large or unpredictable way the results are prone to error and are less reliable.
In the electronics industry, it is often necessary to observe a signal driving a transmission line. Usually it is desirable to terminate the line with its characteristic impedance to prevent reflections due to an impedance mismatch. This characteristic impedance used to terminate the line is commonly 50xcexa9, 75xcexa9, or 100xcexa9.
High frequency measuring instruments, such as those used for signals above 100 MHz, often have 50xcexa9 coaxial inputs. Probes used with these instruments often contain a series resistor very close to the probe tip to attenuate the signal down to a level measurable by the instrument. For example, a 10:1 attenuation may be achieved by placing a 450xcexa9 resistor in series in the probe tip. This 450xcexa9 resistor forms a voltage divider with the 50xcexa9 impedance of the terminated probe cable providing a 10:1 attenuation. The 10:1 attenuation occurs because the voltage is measured across the 50xcexa9 impedance of the terminator.
However, when a 10:1 probe is used to measure a source with a 50xcexa9 impedance, problems occur. Ideally, the source would be terminated with 50xcexa9 right at the point of measurement. However, in this case, the probe acts as a 500xcexa9 impedance at the source since the 450xcexa9 discrete resistor and the 50xcexa9 impedance of the line are in series. This 500xcexa9 is then in parallel with the 50xcexa9 terminating resistor at the source to form a 45.5xcexa9 impedance. This impedance mismatch causes reflections at the measurement point. To eliminate those reflections, a terminating resistor must be used to give a 50xcexa9 impedance when connected in parallel with 500xcexa9. The value of this terminating resistor would be 55.56xcexa9.
As different attenuating probes are used, the resistance needed to keep reflections from occurring changes. Accordingly, there is a need in the art for a means and a method to allow the precise termination of sources at the point of measurement. It is desirable for this device to be very small to minimize the distance between the probe tip and the terminating resistor. Also, it is desirable for the probe tip adapter to be useable on sources of varying impedance and probes of varying impedance and attenuation.
This invention provides a probe tip adapter with an easily interchangeable termination. The probe tip adapter in physically situated between the point of an oscilloscope probe and a transmission line connector, such as a BNC, SMA, or SMB connector. By constructing a probe tip adapter containing a small, bare circuit substrate, this invention allows users to design their own custom probe circuits and assemble them using surface mount (or other) devices to create many different probe circuits for use in many different situations. The user may also keep several unmodified probes in reserve. Finally, users may custom construct their own probe circuit quickly, inexpensively, and easily.